The advantages of surface protection by coating range from longer product life, to improved product reliability, optimal manufacturing cost, opportunities for flexible design and increased recyclability. Applying effective coatings can thus have a wide range of economic impacts in almost all ranges of manufacturing. Coating materials vary from monolithic metals, alloys or ceramics to composites and various combinations of different materials in functionally graded layered structures. Existing coating technologies are classified largely into methods based on chemical and electrophoretic plating, chemical vapor deposition (CVD), physical vapor deposition (PVD), thermal spraying, cladding, dipping, and precursor fusing.
In any of the above techniques, maximum surface protection is achieved by tailoring the coating structure to the highest degree. To this end, the ability to produce coatings with novel microstructures is becoming a major issue in the development of coating technologies. An important category of coating with such novel microstructures is those of intermetallic compounds.
Metal-matrix composites (MMCs) are now used in many areas of manufacturing where combining different properties into one material is critically required. Most current structural applications in the aerospace, automotive and consumer goods industries use MMCs in bulk form. The use of full-volume MMCs however, is not always an optimal choice, particularly in applications where surface properties are of primary importance, but is often practiced because of a lack of technology to apply MMCs to the part surface only.
Metal Matrix Composite coatings on metal substrates impart superior mechanical properties in fatigue and hardness, resistance to abrasion, wear, oxidation, corrosion and high temperatures to the coated part, while the substrate achieves greater strength, stiffness and toughness with respect to the metal matrix. At the same time, coatings provide a more practical and economical option to solid metal-matrix composite products. Such coatings find extensive applications in combustion engine cylinder and piston liners, valves and their seats, jet engine components, electrical contacts, bearing rolls, cams, gears, cutting tool and machine tool surfaces, tank pipes, valves, nozzles, metal working dies, etc.
There are a number of techniques in use for production of MMC coatings, each having its advantages but also its limitations, as they are primarily intended for deposition of conventional materials. Cladding by rolling, extrusion, drawing and explosive techniques, as well as electroplating methods, provide non-fused, structurally discontinuous bonding of the coating to the base metal with relatively low strength. High velocity oxyfuel gas is a combustion spray process used widely for composite coatings, in which semi-molten particles are sprayed at high velocity. Also plasma spray thermal process is a popular current techniques for the production of thin coatings. However, sprayed coatings can be plagued by porosity due to air or oxide entrapment and bond strength sensitivity to the local phase distribution, as well as adhesion problems due to melting and mixing with the base metal.